Treatment chemicals are often injected into subterranean formations containing hydrocarbons to aid in the production of the hydrocarbons. Examples of such treatment chemicals include scale inhibitors, corrosion inhibitors, asphaltene inhibitors, gas hydrate formation inhibitors, and wax deposition inhibitors. Ideally, these treatment chemicals will be retained by the subterranean formation with the treatment chemicals slowly being released from the formation and into the production fluids to provide treatment over an extended period of time. The slow release of the treatment chemicals can provide beneficial effects to the subsurface formation and fluid flow therein, to the well bore and to other downstream flow lines and fluid flow.
As a particular example, scale may cause restriction in flow due to scale deposition in the formation near the well bore, perforations, well bore, flow lines and facilities in hydrocarbon producing systems causing a reduction in the rate of oil and/or gas production. A common method to address scaling problems is to subject the production well to a “squeeze” treatment whereby an aqueous composition comprising scale inhibitors, such as phosphate esters and phosphonates, are introduced into the well, usually under pressure, and “squeezed” or forced into the formation and held there by physical or chemical means. The “squeeze” treatment often needs to be done one or more times a year and constitutes “down time” when no production takes place. Over the year there is a reduction in total production corresponding to the number of down times during the squeeze operations, as well as reduced production as the scaling problem builds up. The net result is frequent well interventions which impact well productivity and field profitability.
Conventionally, a rule of thirds is used when calculating a squeeze operation for scale inhibitors. Generally, a third of the inhibitor will immediately return when the well is put back on production, a third will be ineffective, and a third will be retained in the formation rock and will be slowly released during the life of the squeeze. Accordingly, a great amount of the scale inhibitor is ineffective, increasing the overall cost of the chemicals needed to do the treatment job. Further, because of the low retention efficiency, the time between treatment jobs is unduly short as well.
Calcium ions are often involved in the precipitation or adsorption of phosphonates. It currently believed that the phosphonates react with calcium ions to form a condensed phase that separates from a carrier fluid by adsorption or precipitation and is retained in the formation. A serious problem is getting the calcium and scale inhibitor to interact in the right location. If the scale inhibitor and calcium ions are mixed at a well surface, then precipitation may occur prematurely in the well bore. If the scale inhibitor is injected into the formation and followed by a calcium rich slug of fluid, the mixing in the formation can be very inefficient. The calcium slug tends to displace the inhibitor with little intermixing. Acidic inhibitors can often generate calcium ions by dissolving minerals like calcite that are already in the formation. However, this is not always as controlled as desired and in formations where there is little to no calcite, this is not a good option.
Use of water continuous phases as a carrier for treatment chemicals has shortcomings. Addition of significant amounts of water into a well can, at least temporarily, decrease the permeability of the formation to oil flow. The addition of water may cause clays to swell, again reducing the flow capability of hydrocarbons through a formation. Further, as water is significantly denser than a like volume of oil, low pressure reservoirs may require additional lift support to produce fluids from the hydrocarbon bearing formations.
Use of a generally oil continuous phase as a carrier for treatment chemicals overcomes some problems associated with using water continuous phases. The preparation of water in oil microemulsions to deploy scale inhibitors is described in U.S. Pat. No. 6,581,687 to Collins et al. The disclosure in this patent is hereby incorporated by reference in its entirety. A microemulsion is formed by dispersing an aqueous phase containing a scale inhibitor in an oil phase. The microemulsion is then placed down a production well and into a formation. The formation is then squeezed.
However, like the use of water continuous phases for delivering treatment chemicals to subterranean formations, the method of Collins et al. results in a substantial portion of the introduced treatment chemicals being flushed back with production fluids rather than being retained by rocks of the formation for a slow release over an extended period of time. More efficient retention of the treatment chemicals would extend the life time between squeezes.
The present invention addresses the aforementioned shortcomings in prior methods of chemically treating subterranean formations.